The invention relates to a light-emitting diode of III/V compound semiconductive material having lattice constants heavily dependent on the respective material composition of the mixed crystal, in particular to a GaAsP light-emitting diode.
A light-emitting diode made from a semiconductive material of this type, for example from GaAsP or GaInP is known from the book "Optoelektronik I", Springer Verlag, 1980, pages 139-142. In particular, FIG. 5.5 on page 140 of this publication is a diagram of such a GaAs.sub.0.6 P.sub.0.4 light emitting diode.
The luminescence occurrence in semiconductors made from III/V compounds is based on radiant recombination of electrons of the conduction band with holes from the valence band, with the energy released being emitted in the form of photons. This radiant recombination takes place near or within a p-n junction and supplies almost monochromatic light which is generated by transitions between two almost discrete energy levels. Beforehand, however, the semiconductor must be excited in order to increase the density of the charge carriers above their equivalent concentration. Light-emitting diodes of this type work with minority carrier injection in the case of p-n junctions polarized in the flow direction, so-called injection luminescence.
The known red-emitting GaAs.sub.0.6 P.sub.0.4 light-emitting diodes have an extremely low external quantum efficiency of from approximately 0.4 to 0.6%.
In addition to causes such as non-radiant recombination processes or current flows not contributing to injection, absorption in the semiconductor interior--volume absorption--is an important reason for the low external quantum efficiency in these GaAsP light-emitting diodes. This loss mechanism prior to the emission of radiation from the light-emitting diode chip is a result of the direct energy band gap of GaAs.sub.0.6 P.sub.0.4. To minimize the volume absorption losses, it is necessary in principle to reduce the paths covered by the rays prior to leaving the semiconductor crystal, i.e. the p-n junction should be as close as possible to the surface. For the GaAs diode this means that the p-layer must be kept as thin as possible. The p-n junction cannot, on the other hand, be positioned as close to the surface as wished, as otherwise non-radiant surface recombination will reduce the internal quantum efficiency and consequently also the external quantum efficiency of the light-emitting diode. The thickness of the p-layer must therefore be selected such as to achieve a compromise between volume absorption losses and non-radiant surface recombination.
From the periodical "Journal of Applied Physics", Vol. 49, September 1978, pages 4838 to 4842, particularly FIG. 6, it is known that the volume absorption losses and the losses from surface recombination are minimized when the diffusion depth of the p-n junction is 2.7 times the diffusion length of the minority carriers. Both greater and smaller diffusion depths lead to higher overall losses due to volume absorption and losses from surface recombination respectively.